The present invention relates to a transmission cable structure for transmission of GHz frequency band signals and a connector used for transmission of GHz frequency band signals and more specifically to a connector used for transmission of GHz frequency band signals and a transmission cable structure for transmission of GHz frequency band signals that enables TEM waves to be transmitted between functional circuit blocks without attenuation.
Recently, a variety of problems have arisen concerning long distance communications between functional circuit blocks in line with demands for high-speed communications, such that high-speed communications of 10 Gbps or 100 Gbps on LAN cables using metal wiring is required.
For example, the length of a transmission cable in a system having a pulse clock is limited because a signal, for example a pulse signal having a pulse waveform, transmitting along a transmission cable, deteriorates in accordance with the length of the cable joining the functional circuit blocks.
Generally, applying metal cable for use in high-speed transmissions above 10 Gbps is difficult because the effective distance for transmission of a signal in a metal cable is up to 100 meters, so at this point in time only fiber-optic cable can be used.
In an ideal transmission cable there is no attenuation in the amplitude of an electromagnetic wave, which is a physical object corresponding to a transmission signal like a pulse signal, and no disorder in the waveform of an electromagnetic wave. In other words the electromagnetic wave would be perfectly accommodated by the transmission cable. However, in reality, an electromagnetic wave in a transmission cable suffers resistance loss due to the skin effect of the transmission cable and dielectric loss due to dielectric loss tangent between ground and substrate material. These losses cause attenuation in the amplitude of a transmission signal and distortion of the waveform, what is known as line loss due to RC delay of a transmission cable.
Because resistance loss causes energy dissipation of an electromagnetic wave, this makes the amplitude of a transmission signal smaller but there is no effect of blunting the waveform of the signal. Further, if a transmission cable has the same structure in its entirety, there is actually zero skew of the waveform of a transmission signal. On the other hand, because dielectric loss has frequency characteristics this causes distortion in the waveform of a transmission signal. If, however, the transmission signal is of the same structure in its entirety, this waveform distortion can be forecast, thereby enabling some degree of control.
A number of technologies for transmission cables that enable electromagnetic waves to be efficiently confined therein have been disclosed. For example, one such transmission cable structure is a pair cable structure in which two wires (insulated conductors) are arranged parallel to each other. However, when a plurality of such pair cables are arranged, cross talk arises between pair cables in close proximity.
Moreover, generally, electromagnetic waves are reflected at a connecting point (a mismatching point of impedance) of functional circuit blocks and transmission cables. When there are a plurality of such points of discontinuity, multiple reflections of electromagnetic waves occur at the points of discontinuity. If this leads to resonance of the electromagnetic waves within the transmission cable, the waveform of the transmission signal becomes an extremely complex state in which it is not possible to predict the waveform of it.
The following four methods for preventing such reflection are well-known: (1) inserting damping resistance at the terminal of a driver circuit; (2) making the on resistance of a driver circuit the same as the characteristic impedance of the transmission cable; (3) in a bi-directional bus structure, making the on resistance of driver circuits on both sides the same as that of the on resistance of transmission cables (additionally, including a structure in which damping resistance is inserted in both sides); and (4) attaching terminating resistance matched to the characteristic impedance of the transmission cables, at the terminal of the receiver side of a receiver circuit (this is the ideal method, however, at present, this method is avoided because on current always flows).
As shown in FIG. 1, a conventional driver-receiver circuit (having a driver circuit and a receiver circuit) comprises a single signal line 1003 connecting an output terminal of a driver circuit 1001 and the input terminal of a receiver terminal 1002 as well as a ground line 1004 connecting the ground terminals of those circuits. Although only a single signal line is used here, a transmission cable comprising two lines is required to effectively confine electromagnetic waves. In the case shown in FIG. 1 the ground line or power supply line (not shown in the drawing) fulfills the role of the second line. In this case, however, noise from the ground or power supply and common mode noise enters the transmission cable.
In contrast to this arrangement, to avoid the affect of this common mode noise, in recent years differential circuits have been used substantially in the field of high-speed communications. For example, as shown in FIG. 2, the output terminal of CML type differential circuit 1011 and the input terminal of CML type differential circuit 1012 are mutually connected by transmission cables 1013 and 1014. Further, as shown in FIG. 3, the output terminal of LVDS type differential circuit 1021 and the input terminal of LVDS type differential circuit 1022 are mutually connected by transmission cables 1023 and 1024.
FIG. 4A provides a cross-sectional view of the structure of pair cable 1031 used for a conventional transmission cable and FIG. 4B provides a cross-sectional view of the structure of flat cable 1032 having four pair cables k12 (comprising wires k1 and k2), k34 (comprising wires k3 and k4), k56 (comprising wires k5 and k6) and k78 (comprising wires k7 and k8) arranged adjacently and flatly. The pair cable 1031 is covered with insulating material having relative permittivity of 1.96.
FIG. 5 provides a cross-sectional view of the structure of ground 1042 and pair cables 1041a and 1041b that connect to a conventional differential circuit. This pair cable has a transmission cable structure in which ground 1042 is referenced to match the standard voltage levels between the differential circuits. That is, FIG. 5 shows a conventional differential transmission cable referencing ground (Japanese Patent Publication No. 2002-261843).
Further, among communication cables of conventional technology, the most high-speed wiring cable is TIA/EIA CAT 6 cable having a 1 Gbps transmission speed using an RJ-45 as a connector.